MySQL 8.0

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CREATE TABLE test (
    f1 INT NOT NULL,
    f2 INT NOT NULL,
    f3 INT NOT NULL,
    f4 INT NOT NULL,
    f5 INT NULL,
    UNIQUE ( (CONCAT_WS(CHAR(0), f1,f2,f3,f4,COALESCE(f5, CHAR(0)))) )
);
INSERT INTO test (f1,f2,f3,f4,f5) VALUES (1,1,1,1,1),(1,1,1,1,2),(1,1,1,1,NULL);
INSERT INTO test (f1,f2,f3,f4,f5) VALUES (1,1,1,1,1);
Duplicate entry '1\x001\x001\x001\x001' for key 'test.functional_index'

INSERT INTO test (f1,f2,f3,f4,f5) VALUES (1,1,1,1,NULL);
Duplicate entry '1\x001\x001\x001\x00\x00' for key 'test.functional_index'

SELECT * FROM test;


f1
f2
f3
f4
f5


1
1
1
1
1

1
1
1
1
2

1
1
1
1
null

f1	f2	f3	f4	f5
1	1	1	1	1
1	1	1	1	2
1	1	1	1	null